mac80211: Do not use stack memory with scatterlist for GMAC

ieee80211_aes_gmac() uses the mic argument directly in sg_set_buf() and
that does not allow use of stack memory (e.g., BUG_ON() is hit in
sg_set_buf() with CONFIG_DEBUG_SG). BIP GMAC TX side is fine for this
since it can use the skb data buffer, but the RX side was using a stack
variable for deriving the local MIC value to compare against the
received one.

Fix this by allocating heap memory for the mic buffer.

This was found with hwsim test case ap_cipher_bip_gmac_128 hitting that
BUG_ON() and kernel panic.

Cc: stable@vger.kernel.org
Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>